Prior to the present invention, as shown for example by Goossens et al., U.S. Pat. Nos. 4,242,381 and 4,242,383, and Schroeter et al., U.S. Pat. Nos. 4,210,699, 4,224,378, and 4,239,798, assigned to the same assignee as the present invention, acrylic primers were used on polycarbonate substrates to improve the adhesion characteristics of the filled or unfilled silicone top coats on polycarbonate substrate.
The acrylic primer of the prior art can include thermoplastic acrylic polymers with and without functional groups, for example, hydroxy, carboxyl, amine, etc., and thermoset acrylic polymers. Although improved adhesion of the silicon top-coat on polycarbonate substrates has been effected by using an acrylic primer, the silicone coated polycarbonate articles often did not satisfy the stringent weathering resistance requirements of the automotive industry. A convenient way of determining resistance to weathering which is recognized by the automotive industry is to test silicone-coated thermoplastic slabs in a QUV device, manufactured by the Q-Panel Company of Cleveland, Ohio. This device can be set to consecutive cycles of fluorescent UV light and high humidity at various times and temperatures. The present standards require the ability to pass a severe adhesion test after 600-1600 hours of aging.
The present invention is based on the discovery that improved weathering resistance of silicone-coated, acrylic primed thermoplastic substrates, for example, polycarbonate, can be achieved if there is used a particular combination of adhesion stabilizers in the acrylic primer. More particularly, optimum adhesion after extensive accelerated weathering can be achieved if the silicone top coat is applied onto a thermoplastic substrate primed with an acrylic resin containing a combination of a particular UV absorber and one or more particular light stabilizers as defined hereinafter.